Implants or prostheses are employed for restoring damaged upper and lower extremity bones such as fingers, wrists, elbows, knees and ankles of human patients. These implants are especially useful in the reconstruction of joints which, for example, have been damaged by pathological conditions such as rheumatoid arthritis, degenerative arthritis, aseptic necrosis, and for treating trauma which may have a debilitating effect on articular joints.
Unfortunately, some joint implant designs available currently or described in the past have drawbacks arising from their construction and from the fact that they act merely as spacers for replacing damaged bones. For example, current and past scaphoid and lunate carpal bone replacements are generally undesirable, primarily because they cannot reproduce the normal and vital ligamentous restraints.
Joint replacement designs which rely on mechanical restraint mechanisms of various types (e.g. semi-constrained elbow arthroplasty) and degrees, also fail to simulate or replace ligamentous and capsular restraint along multiple axes. Many arthroplasties attempt to change the native biomechanical properties of the replaced joint instead of reproducing those native properties. For example, current total wrist arthroplasties replace a "link" system with a "hinge" system. Such biomechanical design substitutions have been deemed acceptable only because until now it has not been possible to satisfactorily reconstruct ligaments to joint replacements to achieve the native biomechanical properties of the replaced joint.
There are three types of arthroplasties: 1) unconstrained, 2) semi-constrained and 3) fully constrained. There is a precarious balance between the advantages of unconstrained designs (reduced bone-prosthesis loosening and fracture), and their disadvantages (subluxation and dislocation of the prosthesis and joint). The inherent advantage of a fully constrained device is stability (reduced subluxation and dislocation), whereas, the disadvantage is that most of the vector forces are transferred to the prosthesis-bone interface. This frequently results in loosening or fracture of the bone or of the prosthesis itself.
Semi-constrained devices have historically employed a variety of biomechanical mechanisms attempting to minimize the disadvantages of both unconstrained and fully constrained implants. Examples of semi-constrained implants are total elbow arthroplasties with the so-called "sloppy hinge". The common flaw with all of these current joint replacement designs is the inability to reconstruct and re-attach the replaced joint's vital capsular and ligamentous restraints, which dictate, in large measure, the behavior and stability of the joint.
Every joint has its own unique biomechanical properties which are dictated by the shape of the articulating bones/cartilage, by its function, and most importantly, by its capsular ligamentous three-dimensional restraints. To date, no joint replacement prosthesis has been designed to successfully reconstruct those vital ligamentous/capsular restraints to or through the replacement prosthesis in two or more axes. Restraining a prosthesis in two or more crisscrossing axes, as discussed below, has the mechanical effect of minimizing unwanted transitional and shear forces, while permitting the desired rotational motions necessary for the replaced joint.
This should be distinguished, for example, from implants using two parallel (rather than crisscrossing) channels to hold a "spacer" prosthesis A in position, as illustrated in FIG. 16. The biomechanical result of such an arrangement is the illustrated excessive translation and rotation of the "spacer" along the parallel channels with its potential clinical sequelae of wear debris, chronic instability of the prosthesis, and finally progressive arthritis.
The present invention may apply to any synovial or diarthroidial human joint. However, the preferred application of the invention is to joints whose motion is both quantitatively and qualitatively significant and therefore functionally important.
The definitions of "joints" and "articulations", adopted from Stedman's Medical Dictionary, 1982, pp. 126-7 and p. 737 refer to three types of "articulations": fibrous, cartilogenous, and synovial. The synovial articulation is the preferred application of this invention. A synovial articulation (or diarthrodial joint) is a joint allowing various amounts and types of motion in which the bony surfaces are covered with a layer of hyaline or fibrous cartilage. There is a joint cavity containing synovial fluid and lined with a synovial membrane, reinforced by a fibrous capsule and by ligaments.
In order to better explain the vital importance of the capsular and ligamentous restraints in a synovial joint, and to illustrate the flaws of arthroplasties which do not reconstruct these native restraints, the wrist carpal bones will be discussed below. This discussion will illustrate the anatomy, function and kinematics of the carpus with an emphasis on demonstrating the necessity and unique contribution of the invention as it applies to replacing the scaphoid and lunate carpal bones. The invention, however, is not limited to scaphoid and lunate prostheses but rather extends to all upper and lower extremity arthroplasties in any synovial or diarthroidial joints which are functionally important.
Wrist movement is apportioned between the radiocarpal and midcarpal joints in a very complex manner. The carpus, as discussed above, is biomechanically a link system, not a hinge system, like the knee. Accordingly, it is essential that a carpal implant reproduce the natural synchronous link system motion between it and the adjacent carpal bones to maintain the normal kinematics of the carpus. This serves to preserve the shape of the implant and to prevent wear, fracture, dislocation and particulate synovitis. In other words, synchronous motion of a carpal implant will help maintain normal kinematics of the remaining carpal bones and thus prevent global carpal instability and resultant surrounding arthritis.
The carpal implants most commonly available in the past have been made from silicone. Unfortunately, there are serious potential complications associated with the use of silicone in this and other medical applications. Indeed, since the scaphoid and lunate bones and their restraining ligaments are the most mechanically stressed, they are particularly susceptible to injury and complications. Thus, it is not surprising that it has been commonly reported in the literature that patients who have had silicone carpal implants experience silicone-related complications. These complications included subluxation and dislocation of the prosthesis, fragmentation and fracture of the prosthesis, and finally foreign body giant cell synovitis and focal carpal bone destruction.
Synovitis, mentioned above, is inflammation of the synovial membrane which lines and lubricates the wrist joint. It causes pain and inhibits wrist movement in bone joints. Violation of silicone implants with suturing techniques may contribute to fragmentation, debris and silicone-induced synovitis.
Focal carpal bone destruction is yet another complication which can arise at a later stage as a result of abnormal kinematics and synovitis over an extended period of time. Fragmentation and fracture of silicone implants and the resulting presence of silicone particulate debris results from implant stress related to implant translation subluxation or from implant fracture.
Finally, subluxation is a partial dislocation of the carpal bones. Subluxation and complete implant dislocation are complications which may result from the inherent lack of restraint of current carpal implants to their adjacent carpal bones and to the wrist capsule. In the native carpus, restraint is by way of ligaments and capsule. Thickenings of the palmer and dorsal capsule have been anatomically designated as quasi-discrete ligaments called "extrinsic ligaments" (e.g., radios-capho-capitate ligament). Whereas, those truly discrete interosseous ligaments which directly attach one carpal bone to another are called "intrinsic ligaments" (e.g. scapholunate ligaments). The intrinsic and extrinsic ligaments act dependently to synchronize the complex and balanced intercarpal kinematics. Currently available implants, including those made of both silicone and titanium alloys, do not reproduce the restraining mechanisms of both the intrinsic and extrinsic ligaments, and therefore these prostheses are subject to subluxation and complete dislocation.
To date, a satisfactory technique for reconstruction of intercarpal ligaments and capsular restraints incorporating carpal replacements has not been achieved. While the present invention is uniquely designed to allow the surgeon to accurately and predictably reconstruct the necessary ligamentous restraints and thus prevent the above-mentioned causes of failure, the prior art fails to meet this need.
For example, in U.S. Pat. No. 3,745,590 an implant is disclosed which includes parallel ligamentous elements (defining a single plane) molded into the body of a prosthesis at approximately opposite ends of its top surface. The ligamentous elements are either sutured to adjacent collateral ligaments, tied to the nearest adjacent carpal bone, or tied to an incised ligament or tendon. These ligamentous elements attach the prosthesis along a single axis and the implant is therefore restrained in only one plane. This lack of dual axis restraint may result in subluxation and increased shear.
The carpal metacarpal implant shown in the above-referenced '590 patent includes a stem portion that is integrally formed with the implant body and is adapted to fit into the medullary space in the metacarpal bone to be repaired. This implant includes at least one integral ligamentous element which can be tied or otherwise attached to an adjacent bone, ligament, or tendon. If the implant body includes more than one ligamentous element, the elements extend from a single opening along one edge of the implant body and are similarly tied to adjacent tissues, as described in relation to the first carpal implant above. This embodiment also only restrains the prostheses along a single axis.
Yet another carpal implant is shown in U.S. Pat. No. 4,198,712. This implant includes a stabilizing stem that extends outwardly and generally perpendicularly to the surface of the implant. The stem is adapted to be inserted into an adjacent carpal bone for stabilizing the implant postoperatively. Wires or sutures may be used in conjunction with the stem for temporary fixation and enhanced stabilization of the implant during the early healing process. The wires or sutures are passed through the implant into adjacent carpal bones. The stem and the wires or sutures are intended to restrain the prosthesis along a single axis. Also, as noted earlier, suturing directly into silicone is disfavored as generally it is believed to avulse, trailing silicone debris and potentially leading to silicone synovitis.
Accordingly, an object of the present invention is to provide a method and prostheses for safely replacing upper or lower extremity bone(s) in a joint of a human.
It is another object of the present invention to provide prostheses for replacing upper or lower extremity bone(s) of a joint in which the prostheses are suspended, tethered, and restrained along multiple axes.
It is yet another object of the present invention to provide prostheses for replacing upper or lower extremity joint bone(s) in which the reconstruction of effective restraint means encourages normal global kinematics.
It is a further object of the present invention to provide a method and prostheses for replacing upper or lower extremity joint bone(s) which involves suturing ligamentous means to adjacent capsules and bones, by way of native ligaments, or directly into bone using woven fabric, native capsule, bone-capsule-bone graft, or tendon.
It is yet another object of the invention to provide prostheses with select areas of inset ingrowth surface and/or surface coating to encourage limited ingrowth adhesion to surrounding capsular or ligamentous tissue.
Still another object of the invention is to provide a method for stabilizing a prosthesis while drawing it to and maintaining it directly against an adjacent native capsule to facilitate natural ingrowth adhesion of surrounding capsular or ligamentous tissue into the implant surface to both facilitate anchorage of the implant body directly to native capsular and ligamentous tissue while providing further global stabilization and improved kinematics.
These and other objects and advantages of the invention will appear hereinafter.